In a Code Division Multiple Access (CDMA) communication system, all base stations in all cells may use the same radio frequency for communication. Base stations are uniquely identified in the system by uniquely-assigned spreading codes. Two specified pseudorandom noise (PN) sequences of 215 bits length are used by all the base stations. In a quadrature modulated system, one sequence is used for the in-phase (I) channel spreading of the I channel symbols and the other is used for the quadrature (Q) channel spreading of the Q channel symbols. Mobile stations in the system possess the same two 215 bits length spreading codes and use them for the initial de-spread of the I and Q channels.
The symbols for transmission are spread using a process known as Walsh covering. When in a call, each mobile station is assigned a unique Walsh code by the base site to ensure that transmission to each mobile station within a given cell is orthogonal to transmission to every other mobile station. In addition to traffic channels, each base station broadcasts a pilot channel. The pilot channel is formed by a constant level signal that is covered by Walsh code 0, which consists of all zeros. The pilot channel is commonly received by all mobile stations within range and, among other things, is used by the mobile station for identifying the presence of a CDMA system, identification of initial and delayed rays of communicating and interfering base stations, and for coherent demodulation of the communication channels.
However, the communication channels can be affected by noise, which can be estimated as a signal-to-interference ratio (SIR), also referred to herein as the signal-to-noise ratio (SNR), at the output of the rake receiver. Due to its unique and known nature, the pilot channel can be used to advantage in estimate the channel quality conditions, which can then be used to enhance the communication channels. Channel estimation is an integral part of rake demodulation for Code Division Multiple Access (CDMA) systems such as Third Generation Partnership Project (3GPP) Universal Mobile Telephone System (UMTS) wideband CDMA (WCDMA) and cdma2000 systems. The channel estimator works off of a known pilot sequence to derive a phase reference used for coherent demodulation within each branch (finger) of the rake demodulator.
Referring to FIG. 1, typical CDMA mobile station receivers utilize a rake receiver having three or more (N) independently controlled fingers (branches) which are time aligned to the correct PN sequence phases using knowledge of the pilot channel phases determined by the receiver pilot phase searching element. The rake fingers are normally assigned to the strongest rays received from all communicating base stations as determined by the receiver pilot phase searching element. An incoming quadrature modulated (I+Q) signal is downconverted to a received baseband signal using techniques known in the art. Each finger of the rake receiver operates on the incoming signal with correlators having unique spreading sequences for the pilot and data (traffic) channels. The data correlator detects (despreads) the data information and accumulates it. The pilot correlator detects (despreads) the pilot symbols and accumulates them. The pilot bits can then be used for channel estimation using a channel estimation filter. The filter output, correcting for the estimated channel quality, can then applied to the data symbols using complex conjugation to provide the correction. The channel estimation filter can delay the signal as it is processed so a compensating delay element is used for the data symbols such that the correction is applied coherently. The output from each finger of the rake receiver is then combined after aligning all the signals coherently using individual de-skewer delays for each finger.
A typical channel estimation filter is a finite impulse response (FR) filter that operates by performing a sliding average on the sequence of de-spread pilot symbols, carried on a Common Pilot Channel (CPICH) or on a Dedicated Pilot Channel, which is time multiplexed on a Dedicated Physical Channel (DPCH) in a 3GPP WCDMA system for example. FIG. 2 shows the slot structure for a Common Pilot Channel (CPICH) of a 3GPP WCDMA system. Prior art sliding average channel estimation filters have a delay that varies with the filter length (i.e., delay=one-half filter length). In this case, the channel estimation filter length is set to yield acceptable performance over the full range of expected operating conditions. However, changing the filter bandwidth would most likely cause one or more frame errors. In addition, the delay buffer must be large enough to contain all possible delays that could be encountered, resulting in a very large delay buffer which is costly and takes up much area on an integrated circuit. Moreover, it has been found that Doppler frequency shifts can also affect coherent reception which has not been addressed in the prior art. Although there is a significant amount of prior art on using FIR filters for channel estimation, none of these have a filter length (channel bandwidth) that is varied as a function of Doppler rate.
There is a need for improved channel estimation in a CDMA communication system that further reduces computational complexity and integrated circuit size and cost without sacrificing performance. It would also be of benefit to address Doppler frequency shifts as well as noise.